Current-operated switch

ABSTRACT

A current-operated switch having contacts opened and closed by a pair of bimetallic strips. Individual heating means are provided for the strips to respond to differential operating currents, a difference of one sign closing the contacts and a difference of the opposite sign opening the contacts. The switch draws a constant total current from a supply and is particularly suitable for operation by a differential transistorized amplifier.

2,302,603 11/1942 Davis'et al ..337/96 0 a United States Patent 1191 1111 3,725,670 Davis 1 1 Apr. 3, 1973 s41 CURRENT-OPERATED SWITCH 1,743,053 1 1930 Traver ..337/96 lnventor: Neville Ryland Davis Chignal 2,815,428 12/1957 Pearce ..337/96 X 1 l l Ch l SfiS E z igizgg e m Primary Examiner-Herman J. Hohauser Att0mey-E. F. Wenderoth et al. [22] Filed: Sept. 14, 1971 21 Appl. No.; 180,262 ABSTRACT A current-operated switch having contacts opened [30] Foreign Application Priority D atav and closed by a pair of bimetallic strips. Individual heating means are provided for the strips to respond Sept. 24, 1970 Great Bntam ..45,675/70 to difierential operating currents, a difference of ne 52 us. (:1 ..307/117, 337/96 Sigh clsihg the and a difference of the P- 51 1m. 01. ..,..n0111 35/00 P- Sign P the contacts- The Switch draws a 58 Field of Search ..337/90, 95, 96., 335, 336; constant total current from a pp and is particularly 307/117 112 149 141 1414 1413 suitable for operation by a differential transistorized 1 amplifier. [56] References Cited UNITED STATES PATENTS 7 Claims, 4 Drawing Figures Fa 6,2: :77 3 I; i z:

I r L 5 1 W The invention relates to a current-operated switch.

According to the invention there is provided a current-operated switch having at least one pair of contacts; first and second bi-metallic strips coupled to open and close the contacts; and control means for applying first and second operating currents to heat the first and second bi-metallic strips respectively, the arrangement being such that a differential change of the first and second operating currents in one direction tends to close the contacts and a differential change of the operating currents in the other direction tends to open the contacts. This arrangement allows positive control of both the opening and closing of the contacts and is particularly suitable for operation by both outputs of a differential amplifier. Thus, in a preferred embodiment of the invention the control means for applying the first and second control currents are constituted by, or are controlled by, the differential loads of a differential amplifier. The differential amplifier may be arranged as a flip-flop with feedback coupling the differential outputs to the inputs.

An advantage of a differential amplifier is that the current which it draws is substantially constant. The inherent feedback in such an amplifier is such that any rise in the current in one of the differential loads is counteracted by a corresponding fall in the current in the other differential load. The sum of the currents in the differential loads is therefore substantially constant. This has the practical convenience that Such a control circuit can be run directly from a main supply (usually through a rectifier arrangement) at low voltage by the use of a simple voltage dropping resistor rather than a transformer. Thus, by providing a switch which is particularly suitable for operation by'a. differential amplifier, the present invention allows economies to be effected in many domestic and industrial applications.

Particular application for the invention is found in temperature alarms for bearings or electric motor windings where the contacts may be arranged to light a lamp, sound an alarm or shut down the plant, and the input of the differential amplifier is governed by a temperaturesensitive resistor, thermistor or the like.

In one embodiment of the invention the first and second bimetallic strips are parallel with each other and are fixed together at adjacent ends, the other ends being free and carrying the contacts, one on each strip, the strips being arranged so that on being heated they move in the same direction about their fixed ends. Only if the strips are heated differentially do the contacts come together or move apart. This gives an automatic compensation against theeffect of ambient temperature. The contacts may be normally closed but preferably they are normally open.

In another embodiment of the invention the first and second bimetallic strips are parallel with each other and'are fixed together at adjacent ends and there is provided a switch lever which has a fixed end located near the free ends of the bimetallic strips and a free end which carries a contact of the contact pair, the other contact of the pair being fixed, the bimetallic strips being arranged to bear against the switch lever so that if the first bimetallic strip is heated more than the second bimetallic strip the switch lever is moved in one direction about its fixed end to tend to close the contacts and if the second bimetallic strip is heated'more than the first bimetallic strip the switch lever is moved in the opposite direction about its fixed end to tend to open the contacts.

The invention will further be described with reference to the accompanying drawings, of which:

FIG. 1 is a diagram of a switch embodying the invention;

FIG. 2 is a diagram of another switch embodying the invention;

FIG. 3 is a circuit diagram of a differential amplifier which may be used to control'the switches of FIGS. 1 or 2; and

FIG. 4 is a circuit diagram of a differential amplifier which may be used to control the switches and which is connected as a flip-flop.

Referring to FIG. 1 the switch comprises two bimetal strips 1 and 2 preferably of a nickel iron alloy. The tops of the strips carry a soft iron pad 3 and a magnet 4 respectively. The pad 3 and magnet 4 have contact faces of a material such as silver or silver alloy. The bottoms of the strips are fixed together by an insulating separator 5. A heating coil 6 is provided on strip 1 and a similar heating coil 7 is provided on strip 2. The coils 6 and 7 are connected to a common lead 8. The strips are arranged so that the contacts are normally open and on being heated both strips move in the direction shown by the arrow. Therefore, ambient temperature changes do not affect the spacing of the contacts. However, if coil 7 carries more heating current then coil 6 the contacts will move together and ultimately the magnet 4 will snap the contacts together. If coil 6 carries more current than coil 7 the contacts will move apart. A control circuit including a load L can thereby be made and broken. By proper choice of the magnet 4 and pad 3 and the spacing of the contact gap it is found that adequate snap action can be provided to operate a substantial load. Typically, the switch operates with a power consumption of one quarter to one half of a watt.

Referring now to FIG. 2 there is shown a switch which again has two parallel bimetal strips 1 and 2 carrying respective heating coils 6 and 7. The strips are fixed at ends 9 and 10 and carry projections 11 and 12 at their free ends. The projections bear against a switch lever 13 which is fixed at 14 in the manner of a leaf spring. At its free end the lever 13 carries a soft iron disc 15 with opposed silver contacts at 16 and 17. Fixed silver contacts are provided at 18 and 19 on permanent magnets 20 and 21. The magnets provide a snap action effect for the contacts. It will be seen that the contacts can make and break circuits between leads 22 and 23 and leads 22 and 24 respectively.

The direction of movement of the bimetallic strips on being heated is shown in the Figure by thearrows. Thus, if strip 1 is heated more than strip 2 the lever 13 will be moved to the right in the drawing and ultimately the contacts 17 and 19 will'be made. This results from the application of more heating current to winding 6 than to winding 7. Conversely, if winding 7 receives more current than winding 6 the lever 13 will tend to move to the left to make contacts 16 and 18. Equal changes of temperature in both bimetallic strips because of ambient temperature changes have no net effect on the lever 13.

Referring now to FIG. 3 there is shown an operating circuit suitable for operating the switches of FIGS. 1 or 2. The circuit is a differential amplifier comprising transistors T1 and T2 connected with a common emitter load RL. The bias level at the base of transistor T2 is set by a potentiometer P and the voltage of the base of transistor T1 is determined by temperature since the base is connected to the junction between a thermistor RT and a fixed resistor RF connected across the supply to the circuit. The supply to the circuit is derived through a full wave rectifier D and a voltage dropping resistor RV from the main supply.

The differential loads of the amplifier are resistors R1 and R2 and the operation of the amplifier is such that the sum of the currents in resistors R1 and R2 is substantially constant. Thus, if the response of the thermistor causes the current in resistor R1 to rise then a corresponding fall in the current in the resistor R1 to operate as a flip-flop. In the circuit of FIG. 4 the voltage of the differential loads R1 and R2 are used to operate further transistors T3 and T4 which have collector loads R3 and R4. Loads R3 and R4 may be identified with heating elements 6 and 7 of FIG. 1 or in driving the different kinds of switch. The flip-flop action is determined by a regenerative coupling through resistor R from the collector of transistor T4 to the base of transistor T1 and through a resistor R6 from the collector of transistor T3 to the base of transistors T2. A capacitor C is provided across the supply to the circuit to smooth out transients. Also, there is provided a Zener diode Z for regulating the supply voltage. For example, the Zener diode may have a breakdown voltage of 30 volts to protect the circuit which normally operates at an applied voltage of 20 volts R.M.S.

It is to be understood that the invention is not restricted to the details of the specific embodiments described above with reference to'the drawings. For example, the input to the differential amplifier may be determined by any variable representative of an effect which it is desired to control. The FIG. 2 embodiment may be modified by pivoting the lever 13 at 14. Magnets 2'0 and 21 may be omitted and the switch used as a deviation indicator by using lever 13 as a pointer.

Switch action at selected upper and lower limits of diviation may be achieved by making the positions of contacts 18 and 19 adjustable. In the FIG. 4 arrangement the capacitor C and/or the Zener diode Z may be omitted.

I claim:

1. A current-operated switch in combination with a differential amplifier having differential loads, the current operated switch comprising at least one pair of contacts and first and second bimetallic strips coupled to open and close the contacts, the differential loads of the differential amplifier being connected to constitute heating means for heating respectivel the bimetallic strips and the strips being arranged so t at a differential change in one direction of the currents in the differential loads tends to close the contacts and a differential change in the other direction of the currents in the differential loads tends to open the contacts.

2. A current-operated switch as claimed in claim 1 wherein the first and second bimetallic strips are parallel with each other and are fixed together at adjacent ends, the other ends being free and carrying the contacts, one on each strip, the strips being arranged so that on being heated they move in the same direction about their fixed ends.

3. A current-operated switch as claimed in claim 1 wherein the first and second bimetallic strips are parallel with each other and are fixed together at adjacent ends and there is provided a switch lever which has a fixed end near the free ends of the bimetallic strips and a free end which carries a contact of the contact pair, the other contact of the pair being fixed, the bimetallic strips being arranged to bear against the switch lever so that if the first bimetallic strip is heated more than the second bimetallic strip the switch lever is moved in one direction about its fixed end to tend to close the contacts and if the second bimetallic strip is heated more than the first bimetallic strip the switch lever is moved in the opposite direction about its fixed end to tend to open the contacts. 1

4. A current-operated switch as claimed in claim 3 wherein there is a second pair of contacts, of which one contact is fixed and the other is carried on the free end of the switch lever, the contacts being arranged so that as one pair tends to close the other pair tends to open.

5. A current-operated switch as claimed in claim 1 wherein the contacts of the at least one pair of contacts are magnetically attracted, the magnetic attraction giving a snap action effect to the contacts.

6. The combination claimed in claim 1 wherein the differential amplifier is connected as a flip-flop circuit with feedback coupling the differential outputs to the amplifier input.

7. The combination claimed in claim 1 wherein the differential amplifier derives operating power from a rectifier arrangement which is connected to a mains supply through a voltage-dropping resistor. 

1. A current-operated switch in combination with a differential amplifier having differential loads, the current operated switch comprising at least one pair of contacts and first and second bimetallic strips coupled to open and close the contacts, the differential loads of the differential amplifier being connected to constitute heating means for heating respectively the bimetallic strips and the strips being arranged so that a differential change in one direction of the currents in the differential loads tends to close the contacts and a differential change in the other direction of the currents in the differential loads tends to open the contacts.
 2. A current-operated switch as claimed in claim 1 wherein the first and second bimetallic strips are parallel with each other and are fixed together at adjacent ends, the other ends being free and carrying the contacts, one on each strip, the strips being arranged so that on being heated they move in the same direction about their fixed ends.
 3. A current-operated switch as claimed in claim 1 wherein the first and second bimetallic strips are parallel with each other and are fixed together at adjacent ends and there is provided a switch lever which has a fiXed end near the free ends of the bimetallic strips and a free end which carries a contact of the contact pair, the other contact of the pair being fixed, the bimetallic strips being arranged to bear against the switch lever so that if the first bimetallic strip is heated more than the second bimetallic strip the switch lever is moved in one direction about its fixed end to tend to close the contacts and if the second bimetallic strip is heated more than the first bimetallic strip the switch lever is moved in the opposite direction about its fixed end to tend to open the contacts.
 4. A current-operated switch as claimed in claim 3 wherein there is a second pair of contacts, of which one contact is fixed and the other is carried on the free end of the switch lever, the contacts being arranged so that as one pair tends to close the other pair tends to open.
 5. A current-operated switch as claimed in claim 1 wherein the contacts of the at least one pair of contacts are magnetically attracted, the magnetic attraction giving a snap action effect to the contacts.
 6. The combination claimed in claim 1 wherein the differential amplifier is connected as a flip-flop circuit with feedback coupling the differential outputs to the amplifier input.
 7. The combination claimed in claim 1 wherein the differential amplifier derives operating power from a rectifier arrangement which is connected to a mains supply through a voltage-dropping resistor. 